Very early acetaldehyde production by industrial Saccharomyces cerevisiae strains: a new intrinsic character

Study on the changes during the fermentation of the wine prepared from palm (Phoenix sylvestris) sap

The sugary sap of different palm trees is fermented to create palm wine, an alcoholic beverage. This work was aimed at studying the changes that occur during the fermentation process of wine made from the sap of the wild date palm species Phoenix sylvestris. At first, the best age of the palm tree was determined by observing total soluble solid and sap yield for 24 h and was found to be middle-aged palm plants (15-40 years old). Pure wine yeast (Saccharomyces cerevisiae SC22) and a natural starter culture were added to the palm saps, adjusting the total soluble solid (TSS) to 21.5° brix (°Bx). Total titratable acidity, pH, volatile acidity, reducing sugar, non-reducing sugar, total sugar, alcohol content, ester content, and aldehyde contents were the parameters under investigation. The statistical analysis showed significant (p ≤ 0.05) changes in the physico-chemical and volatile constituents of palm sap during the fermentation process in both systems. Sensory evaluation revealed that palm wine fermented with pure yeast culture was significantly superior to natural, spontaneously fermented wine. The acceptability test showed that the ideal characteristics of palm wine are cloudy in appearance, fruity in aroma, and sweet in taste.

Ethanolic Fermentation of Rye Mashes: Factors Influencing the Formation of Aldehydes and Process Efficiency

High concentrations of aldehydes may result in poor-quality agricultural distillate. We investigate the influence of the method of mash preparation, the initial pH of the mashes, and different yeast strains on the fermentation efficiency and concentration of aldehydes from C2 (acetaldehyde) to C7 (enanthaldehyde) in rye mashes. The tested factors were revealed to have a differentiated influence on both the process efficiency and the concentrations of aldehydes, especially in the case of the dominant acetaldehyde. Mashes obtained from steamed rye grain showed significantly higher fermentation efficiencies than those prepared by the pressureless method. Increasing the pH of the sweet mashes from 4.5 to 6.0 resulted in significantly higher concentrations of acetaldehyde, especially in the case of steamed rye grain. Moreover, an increase in the concentrations of other aldehydes, i.e., from C3 (propionaldehyde) to C5 (valer- and isovaleraldehyde) was observed. A high fermentation efficiency and the lowest acetaldehyde concentrations were obtained from steamed rye mashes with an initial pH of 4.5, fermented using the yeast strains DistilaMax GW and DistilaMax HT. DistilaMax HT yeast also provided a relatively low concentration of acetaldehyde in mashes with an initial pH in the range of 4.5–5.5 prepared by the energy-saving pressureless method.

QTL mapping: an innovative method for investigating the genetic determinism of yeast-bacteria interactions in wine

The two most commonly used wine microorganisms, Saccharomyces cerevisiae yeast and Oenococcus oeni bacteria, are responsible for completion of alcoholic and malolactic fermentation (MLF), respectively. For successful co-inoculation, S. cerevisiae and O. oeni must be able to complete fermentation; however, this relies on compatibility between yeast and bacterial strains. For the first time, quantitative trait loci (QTL) analysis was used to elucidate whether S. cerevisiae genetic makeup can play a role in the ability of O. oeni to complete MLF. Assessment of 67 progeny from a hybrid S. cerevisiae strain (SBxGN), co-inoculated with a single O. oeni strain, SB3, revealed a major QTL linked to MLF completion by O. oeni. This QTL encompassed a well-known translocation, XV-t-XVI, that results in increased SSU1 expression and is functionally linked with numerous phenotypes including lag phase duration and sulphite export and production. A reciprocal hemizygosity assay was performed to elucidate the effect of the gene SSU1 in the SBxGN background. Our results revealed a strong effect of SSU1 haploinsufficiency on O. oeni's ability to complete malolactic fermentation during co-inoculation and pave the way for the implementation of QTL mapping projects for deciphering the genetic bases of microbial interactions. KEY POINTS: • For the first time, QTL analysis has been used to study yeast-bacteria interactions. • A QTL encompassing a translocation, XV-t-XVI, was linked to MLF outcomes. • S. cerevisiae SSU1 haploinsufficiency positively impacted MLF by O. oeni.

Influence of pre-fermentative addition of aqueous solution tannins extracted from oak wood (Quercus petraea) on the composition of Grillo wines

In this research, the chemical characterization of fixed and volatile compounds of two different tannins in aqueous solution (Pratiko® L-Harvest and L-Fruit) extracted from oak wood, has been studied. The influence of the above tannins, at different concentrations, on the alcoholic fermentation kinetics and on the composition and sensorial characteristics of a white wine were then evaluated. The wines added tannins in aqueous solution compared to control wines showed significant differences in fixed compounds (colloids, polyphenols and ellagitannins) and volatile compounds (phenolic aldehydes, volatile phenols, furanic and piranic compounds). The differences of aqueous solution tannins extracted from oak wood were partly due to the drying/maturing and roasting methods used in barrel production. Alcoholic fermentation was partially facilitated by the addition of tannins in aqueous solution. The wines obtained showed a higher content of ethyl esters of medium-chain fatty acids (from 22 to 31%) and, in some cases, higher acetate alcohols (from 15 to 28%), relevant to the olfactory sensations provided to the wines. The tannins added to the must before fermentation also made it possible to obtain an additional supply of polyphenols (from 25 to 85%) able to induce more complex sensory profiles in the wines, with increased persistent taste notes.

Comprehensive study of the dynamic interaction between SO2 and acetaldehyde during alcoholic fermentation

In this work, we focused on the effect of the initial content of SO₂ in synthetic grape juice on yeast metabolism linked to the production of acetaldehyde. Lengthening of the lag phase duration was observed with an increase in the initial SO₂ content. Nevertheless, an interesting finding was a threshold value of an initial SO₂ content of 30 mg L^-1 in the juice led to equilibrium between intracellular SO₂ diffusion and SO₂ production from the sulfate pool by yeast. The ratios of free and bound acetaldehydes were measured during fermentation, and the maximum accumulation of free acetaldehyde was observed when SO₂ concentration equilibrium between diffusion and production was reached in the fermenting juice. Moreover, it was observed that SO₂ addition resulted in significant changes in the synthesis of aroma compounds. Production of volatile molecules related to sulfur metabolism (methionol) was changed. But, more surprisingly, synthesis of some volatile carbon compounds (diacetyl, isoamyl alcohol, isobutyl alcohol, phenyl ethanol and their corresponding esters) was also altered because of major disruptions in the NADPH/NADP⁺ redox equilibrium. Finally, we demonstrated that acetaldehyde bound to SO₂ could not be metabolized by the yeast during the time course of fermentation and that only free acetaldehyde could impact metabolism.

SSU1 Checkup, a Rapid Tool for Detecting Chromosomal Rearrangements Related to the SSU1 Promoter in Saccharomyces cerevisiae: An Ecological and Technological Study on Wine Yeast

Chromosomal rearrangements (CR) such as translocations, duplications and inversions play a decisive role in the adaptation of microorganisms to specific environments. In enological Saccharomyces cerevisiae strains, CR involving the promoter region of the gene SSU1 lead to a higher sulfite tolerance by enhancing the SO₂ efflux. To date, three different SSU1 associated CR events have been described, including translocations XV-t-XVI and VIII-t-XVI and inversion inv-XVI. In the present study, we developed a multiplex PCR method (SSU1 checkup) that allows a rapid characterization of these three chromosomal configurations in a single experiment. Nearly 600 S. cerevisiae strains collected from fermented grape juice were genotyped by microsatellite markers. We demonstrated that alleles of the SSU1 promoter are differently distributed according to the wine environment (cellar versus vineyard) and the nature of the grape juice. Moreover, rearranged SSU1 promoters are significantly enriched among commercial starters. In addition, the analysis of nearly isogenic strains collected in wine related environments demonstrated that the inheritance of these CR shapes the genetic diversity of clonal populations. Finally, the link between the nature of SSU1 promoter and the tolerance to sulfite was statistically validated in natural grape juice containing various SO₂ concentrations. The SSU1 checkup is therefore a convenient new tool for addressing population genetics questions and for selecting yeast strains by using molecular markers.

Changes in the Concentration of Carbonyl Compounds during the Alcoholic Fermentation Process Carried out with Saccharomyces cerevisiae Yeast

The aim of the study was to determine the influence of the source material and the applied S. cerevisiae strain on the concentrations of carbonyl fractions in raw spirits. Acetaldehyde was the most common aldehyde found, as it accounted for 88-92% of the total amount of aldehydes. The concentration of acetaldehyde in maize, rye and amaranth mashes was highly correlated with fermentation productivity at a given phase of the process, and reached its highest value of 193.5 mg/l EtOH in the first hours of the fermentation, regardless of the yeast strain applied. The acetaldehyde concentration decreased over the time with the decreasing productivity, reaching its lowest value at the 72nd hour of the process. The final concentration of acetaldehyde depended on the raw material used (ca 28.0 mg/l EtOH for maize mashes, 40.3 mg/l EtOH for rye mashes, and 74.4 mg/l EtOH for amaranth mashes). The effect of the used yeast strain was negligible. The overall concentration of the analyzed aldehydes was only slightly higher: ca 30.3 mg/l EtOH for maize mashes, 47.8 mg/l EtOH for rye mashes, and 83.1 mg/l EtOH for amaranth mashes.

Unraveling the Mechanisms for Low-Level Acetaldehyde Production during Alcoholic Fermentation in Saccharomyces pastorianus Lager Yeast

Acetaldehyde is produced by yeast during alcoholic fermentation, and its modification greatly affects beer flavor and quality. In the current study, we analyzed two yeast strains with a low level of acetaldehyde to reveal the potential mechanism underpinning the desirable low acetaldehyde production by these strains. We demonstrated that high alcohol dehydrogenase (ADH) activity and high NADH availability were the dominant factors for the low level of acetaldehyde in the fermentation liquor at the end of fermentation. High ADH activity resulted in reduced accumulation of acetaldehyde during the cell growth phase by increasing the flux to ethanol, whereas high NADH availability (in the cytosol or mitochondria) enhanced acetaldehyde reduction at the later phase of main fermentation. Furthermore, NADH availability is a more useful target trait than ADH activity for constructing yeast strains with a low level of acetaldehyde for industrial applications in terms of flavor contribution and unaltered fermentation period.

Comprehensive Study of the Evolution of the Gas-Liquid Partitioning of Acetaldehyde during Wine Alcoholic Fermentation

Determining the gas-liquid partitioning ( K_i) of acetaldehyde during alcoholic fermentation is an important step in the optimization of fermentation control with the aim of minimizing the accumulation of this compound, which is responsible for the undesired attributes of green apples and fresh-cut grass in wines. In this work, the effects of the main fermentation parameters on the K_i of acetaldehyde were assessed. K_i values were found to be dependent on the temperature and composition of the medium. A nonlinear correlation between the evolution of the K_i and fermentation progress was observed, attributable to the strong retention effect of ethanol at low concentrations, and it was demonstrated that the partitioning of this specific molecule was not influenced by the CO₂ production rate. A model was developed that quantifies the K_i of acetaldehyde with a very accurate prediction, as the difference between the observed and predicted values did not exceed 9%.

Combined effect of the Saccharomyces cerevisiae lag phase and the non-Saccharomyces consortium to enhance wine fruitiness and complexity

Non-Saccharomyces (NS) species that are either naturally present in grape must or added in mixed fermentation with S. cerevisiae may impact the wine's chemical composition and sensory properties. NS yeasts are prevailing during prefermentation and early stages of alcoholic fermentation. However, obtaining the correct balance between S. cerevisiae and NS species is still a critical issue: if S. cerevisiae outcompetes the non-Saccharomyces, it may minimize their impact, while conversely if NS take over S. cerevisiae, it may result in stuck or sluggish fermentations. Here, we propose an original strategy to promote the non-Saccharomyces consortium during the prefermentation stage while securing fermentation completion: the use of a long lag phase S. cerevisiae. Various fermentations in a Sauvignon Blanc with near isogenic S. cerevisiae displaying short or long lag phase were compared. Fermentations were performed with or without a consortium of five non-Saccharomyces yeasts (Hanseniaspora uvarum, Candida zemplinina, Metschnikowia spp., Torulaspora delbrueckii, and Pichia kluyveri), mimicking the composition of natural NS community in grape must. The sensorial analysis highlighted the positive impact of the long lag phase on the wine fruitiness and complexity. Surprisingly, the presence of NS modified only marginally the wine composition but significantly impacted the lag phase of S. cerevisiae. The underlying mechanisms are still unclear, but it is the first time that a study suggests that the wine composition can be affected by the lag phase duration per se. Further experiments should address the suitability of the use of long lag phase S. cerevisiae in winemaking.

An original method for producing acetaldehyde and diacetyl by yeast fermentation

In this study a natural culture medium that mimics the synthetic yeast peptone glucose medium used for yeast fermentations was designed to screen and select yeasts capable of producing high levels of diacetyl and acetaldehyde. The presence of whey powder and sodium citrate in the medium along with manganese and magnesium sulfate enhanced both biomass and aroma development. A total of 52 yeasts strains were cultivated in two different culture media, namely, yeast peptone glucose medium and yeast acetaldehyde-diacetyl medium. The initial screening of the strains was based on the qualitative reaction of the acetaldehyde with Schiff's reagent (violet color) and diacetyl with Brady's reagent (yellow precipitate). The fermented culture media of 10 yeast strains were subsequently analyzed by gas chromatography to quantify the concentration of acetaldehyde and diacetyl synthesized. Total titratable acidity values indicated that a total titratable acidity of 5.5 °SH, implying culture medium at basic pH, was more favorable for the acetaldehyde biosynthesis using strain D15 (Candida lipolytica; 96.05 mg L⁻¹ acetaldehyde) while a total titratable acidity value of 7 °SH facilitated diacetyl flavor synthesis by strain D38 (Candida globosa; 3.58 mg L⁻¹ diacetyl). Importantly, the results presented here suggest that this can be potentially used in the baking industry.

New precursor of 3-mercaptohexan-1-ol in grape juice: thiol-forming potential and kinetics during early stages of must fermentation

Two volatile thiols, 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA), are key aroma impact compounds in many young white wines, especially of the variety Sauvignon blanc (SB). Although great effort has been invested to identify their precursors in recent years, the origin of the majority of 3MH and 3MHA generated during wine fermentation still cannot be explained. Here we demonstrate that supplying an external source of hydrogen sulfide to grape juice hugely increases its thiol-forming potential. We further describe the discovery of (E)-2-hexen-1-ol as an additional new thiol precursor and demonstrate that it possesses, together with (E)-2-hexenal, an immense thiol-forming potential during fermentation. Both C6-compounds are extremely rapidly metabolized by yeast during the first hours after inoculation, even under commercial conditions, and can be interconverted during this phase depending on their initial concentration in the grape juice. Spiking grape juice with additional acetaldehyde greatly enhanced the (E)-2-hexen-1-ol to (E)-2-hexenal conversion rate. Delaying the metabolization of the two unsaturated C6-thiol precursors by yeast, at the same time as increasing hydrogen sulfide production early in fermentation, opens up a great opportunity to tap into this enormous potential 3MH and 3MHA source in grape juice and extends the possibility of thiol production to other non-grape-based alcoholic beverages as well.

Improved sample preparation and rapid UHPLC analysis of SO2 binding carbonyls in wine by derivatisation to 2,4-dinitrophenylhydrazine

Sulphur dioxide (SO2) is essential for the preservation of wines. The presence of SO2 binding compounds in musts and wines may limit sulphite efficacy leading to higher total SO2 additions, which may exceed SO2 limits permitted by law and pose health risks for sensitive individuals. An improved method for the quantification of significant wine SO2 binding compounds is presented that applies a novel sample treatment approach and rapid UHPLC separation. Glucose, galacturonic acid, alpha-ketoglutarate, pyruvate, acetoin and acetaldehyde were derivatised with 2,4-dinitrophenylhydrazine and separated using a solid core C18 phase by ultra high performance liquid chromatography. Addition of EDTA to samples prevented de novo acetaldehyde formation from ethanol oxidation. Optimised derivatisation duration enhanced reproducibility and allowed for glucose and galacturonic acid quantification. High glucose residues were found to interfere with the recovery of other SO2 binders, but practical SO2 concentrations and red wine pigments did not affect derivatisation efficiency. The calibration range, method accuracy, precision and limits of detection were found to be satisfactory for routine analysis of SO2 binders in wines. The current method represents a significant improvement in the comprehensive analysis of SO2 binding wine carbonyls. It allows for the quantification of major SO2 binders at practical analyte concentrations, and uses a simple sample treatment method that prevents treatment artifacts. Equipment utilisation could be reduced by rapid LC separation while maintaining analytical performance parameters. The improved method will be a valuable addition for the analysis of total SO2 binder pools in oenological samples.